Thing is you entered their home without their permission, so trespass at best.
And although you have all the animal lovers' understanding, it's ultimately not about the cat, it's about their rights as legal occupants.
The biggest issue is that you (correctly) contacted the police but then (incorrectly) overrode their suggestion that you cannot legally do anything.
What you did was illegal and it's not too much of a stretch to think they will be beyond furious.
I would be, and would take legal action. This is someone's home and you trespassed. That's pretty serious, it's really violating. You saw all their stuff, you know their home but they didn't invite you.
It's really in their hands how it pans out, some pp say they'd laugh at it, so you might be lucky. However if it was my home I'd have you prosecuted I'm afraid.
Please don't flame, not everyone is an animal lover, that's all. I'd have preferred you to leave the cat in there and deal with the mess later, rather than have complete strangers in my place.
I can hear the knives coming out at that already, hey ho...